You can specify if ratsnest processes polygons or not :
"set poly on"
"set poly off"
in the commandline, again (and without ")

Once a polygon have been rendered, there is no easy way to turn it off again, but there are workarounds :
a) drag one edge a bit away, then back again, then refresh display (zoom out, etc).
c) close the .BRD only and the open it again from the schematic editor. DO save changes if prompted so, otherwise board and schematic will be inconsistent and will stop being updated synchronuously (the so-called back-annotation).